Trouble installing a script file of an addon (threejs export)
export_threejs.py is only part of an addon, you need all of files or it won't work.

https://github.com/mrdoob/three.js/tree/master/utils/exporters/blender/2.65/scripts/addons/io_mesh_threejs

A __init__.py is crucial, because it contains the bl_info dict to make it a Blender addon, and it loads the other files.

In your ...\scripts\addons\ folder, there needs to be a subfolder (e.g. called io_mesh_threejs) with all 3 files of the addon inside.

In order to use Blender's Addon Install from file... feature for addons with multiple files, the mentioned folder with the scripts (io_mesh_threejs) needs to be zipped and you select that zip-file.